negative WBS commitment only when Purchase requisition creation

Hi Gurus,

i am experiencing a weird situation. when a Purchase requisition is created with a WBS then the commitment that appears for that G/L and WBS in the report S_ALR_87013542 is negative. nevertheless when the Preq is converted into a PO then the commitment is balanced and it is appearing blank. Do you know why it is appearing a negative commitment when the Preq is created? should not it be positve one?

thanks and regards

Ivan